1. Field of the Invention
The present invention relates to a display panel, and particularly to a manufacturing method for forming active elements formed on an insulating substrate constituting the display panel.
The present invention is suitable for manufacturing a display device using an insulating substrate with active elements formed in a strip-shaped poly-crystalline semiconductor film, obtained by reforming an amorphous or granular poly-crystalline semiconductor film formed on one of the main surfaces of the insulating substrate constituting the display device so as to expand crystal grains into a substantially strip-like shape by irradiating the film with laser light (also referred to as laser beam).
2. Description of the Related Art
In current display devices, which are so-called display panels and flat displays such as liquid crystal and organic electroluminescence display units, an image is formed by switching active elements (thin film transistor: TFT) of a plurality of pixel circuits consisting of an amorphous or poly-crystalline silicon film on a substrate of glass, fused quartz or the like with an insulating film interposed therebetween. A substrate on which pixel circuits are formed is referred to as an active matrix substrate, TFT substrate, or is simply referred to as an active panel or a substrate.
If peripheral circuit (hereinafter also referred to as pixel driver circuits and the like or simply as driver circuits) including a pixel driver circuit for driving pixel circuits can be simultaneously formed on such a substrate, a dramatic reduction in production costs and improvement in reliability are expected. However, when an amorphous silicon is used as a silicon semiconductor film (hereinafter also referred to as silicon thin film or silicon film) constituting an active layer of a thin film transistor constituting a pixel driver circuit and the like, the performance of the thin film transistor, of which typical example is mobility, is low. Thus, it is difficult to manufacture a pixel driver circuit and the like, for which high speed and high function are required, from an amorphous silicon.
In order to manufacture such a high-speed and high-function pixel driver circuit and the like, a high-mobility thin film transistor is required. To realize the high-mobility thin film transistor, it is necessary to improve the crystallinity of the silicon thin film. As a method for improving the crystallinity, excimer laser annealing has been getting a lot of attention. According to this excimer laser annealing (ELA) method, mobility is improved by irradiating an amorphous silicon film formed on an insulating substrate of glass or the like with an insulating film interposed therebetween with excimer laser so that the amorphous silicon film is transformed into a granular poly-crystalline silicon film.
However, in the granular poly-crystalline silicon film obtained by the ELA method, the grain size is about several tens to hundreds of nanometers. Thus, the granular poly-crystalline silicon film is deficient in performance to be applied to a pixel driver circuit or the like for driving a pixel transistor in a display device at a high speed.
As a solution to this problem, Patent document 1 discloses a method of forming so-called strip-like crystals in which a polysilicon portion is scanned and irradiated with a time-modulated continuous-wave laser beam (laser beam) condensed into a linear form (a rectangular form in which the width of the long axis is extremely greater than that of the short axis) at a high speed in the direction which crosses the longitudinal direction (long axis) of the linear form so that crystals are laterally grown in the scanning direction. According to this method, the entire surface of the substrate is poly-crystallized by the excimer laser annealing, and then only a region where driver circuits are formed is scanned by the above-mentioned laser beam condensed into a linear form in the direction which coincides with the current path (drain-source direction) of the formed transistor so that crystal grains are grown in the scanning direction (lateral direction) in the form of strips. As a result, the mobility is greatly improved by the absence of crystal grain boundaries which traverse the current path or by reforming the crystal grains into minute strip-like crystalline silicon. This method is herein referred to as SELAX (Selectively Enlarging LAser X'tallization) method.
In the SELAX method, to obtain a uniform strip-shaped crystalline silicon film, the laser beam is shaped into a linear form whose intensity distribution along the above long axis of the laser beam has a flat top and width of the short axis is reduced to a few microns. At this time, the width of the short axis of the linear-shaped beam is ununiform in some cases due to the influences of the lens aberration of the laser optical system and the like. Since there have been no techniques for quantitatively evaluating the beam profiles of laser beams which can deal with this situation, a variation in the crystal quality occurs. This is one of the factors that limit an improvement in the yield of the products. To mitigate the variation in the crystal quality in the SELAX step and to mitigate a decrease in the yield of the products, it is necessary to establish a method of evaluating the beam profile of the laser beam which has a direct correlation with the quality of the crystalline film, and to control the beam profile of the laser beam based on the evaluation.
An example of the documents which disclose known techniques relating to evaluation of the beam profile of laser beam is Patent document 2. Patent document 2 discloses the ELA crystallization, in which the beam profile of the laser beam is controlled by calculating energy density by detecting a part of the laser power and a part of the width of the short axis of the beam profile of the laser beam all the time and feedback-controlling a transmittance adjusting mechanism so that the energy density is kept constant all the time.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2003-124136
[Patent Document 2]
Japanese Unexamined Patent Publication No. 2001-102323